A multiple-input multiple-output (MIMO) system is defined as a system that improves data communication efficiency by the use of multiple transmit antennas and multiple receiving antennas. The MIMO system may be realized using a MIMO scheme such as a spatial multiplexing and a spatial diversity. According to the spatial multiplexing, different streams are concurrently transmitted through the multiple transmit antennas, and thus fast transmission can be achieved without increase of system bandwidth. According to the spatial diversity, same streams are transmitted through the multiple transmit antennas to obtain diversity.
In order to reproduce a signal transmitted from a transmitter, channel estimation has to be carried out by a receiver. Channel estimation is defined as a process in which a distorted signal is restored by compensating for signal distortion due to the fading. In general, for the channel estimation, reference signals which are known by both the transmitter and the receiver are required.
The reference signals may be allocated using either the first scheme in which the reference signals are allocated over the entire frequency band or the second scheme in which the reference signals are allocated over a part of the frequency band. The reference signals are further densely allocated in the first scheme rather than the second scheme. The channel estimation can be further accurately performed when the first scheme is used. On the other hand, a higher data transfer rate can be achieved in the second scheme rather than the first scheme. In the second scheme, the reference signals are scarcely allocated, and thus the channel estimation may degrade.
In the MIMO system, multiple channels are independently provided for multiple antennas. The reference signals need to be allocated in consideration of the multiple channels. In addition, the MIMO system may operate in either a single-codeword mode or a multiple-codeword mode according to a rank. The number of reference signals may increase along with the increase of the number of transmit antennas, and thus the number of antennas may increase. But, this may adversely affect the data transmission rate.
Also, if the multiple antennas are used, there is a need for considering each transmit power to each antenna. The reference signals are generally allocated with higher power than data. If the reference signals are allocated without the consideration of the transmit power, there is a difficulty that same power cannot be allocated to each antennas for applying transmit diversity or precoding.
Therefore, there is a need for a method in which the reference signals can be effectively allocated in consideration of the transmit power in the multiple-input multiple-output (MIMO) system.